The broiler pan you linked to is made of porcelain-enameled steel (this is super common, especially for broiler pan sets that come with ranges), so it physically cannot be seasoned.
Like Darren said, you can either scrub until your arm falls off, or cover it with aluminum foil. If the dirty pan really bugs you, you can always
buy a new one for around 20 bucks and vow to keep the new one clean on an ongoing basis to prevent it from getting just as funky-looking as your old one